Position information services such as navigation and an information search service are provided, for example, using Augmented Reality (AR) on a portable terminal equipped with a camera, a Global Positioning System (GPS), an acceleration sensor, an electronic compass, and the like. Displaying an actual landscape, shoot by a camera, with various pieces of information superimposed thereon by the AR makes it possible to provide easy-to-understand navigation and easy-to-understand information search services. For example, by displaying multiple pieces of information such as merchandises and menus of surrounding shops in a downtown in a superimposed manner, the conditions of the surrounding shops can easily be grasped and a favorite shop can be searched at once.
However, it was difficult to use the AR on a portable terminal inside of a building. Further, even at the outside of a building, it was difficult to use the AR on a portable terminal at a canyon of high-rise building and the like. This is because, at the aforementioned sites such as the inside of a building, the positioning by the GPS cannot be performed or is very difficult and the accuracy of an electronic compass is greatly decreased. Hence, techniques for measuring the position and the direction by autonomous navigation have been developed. The autonomous navigation is a technique for estimating the movement of a user by combining plural sensors such as an acceleration sensor, a gyro sensor, and a magnetic sensor and calculating the position and the moving direction (azimuth) from a reference position. For example, this technique makes it possible to calculate the relative position and the azimuth from a reference position (for example, entrance of shop or the like) even at sites (for example, inside of a building, a canyon of high-rise building, and the like) where the acquisition of the position and the azimuth is difficult by methods such as the GPS and the electronic compass and to enable an AR service.
However, since the autonomous navigation calculates the current position and the current azimuth by adding the measurement values of the changes of the position and the moving direction from a reference position (start of measurement), there is a possibility that measurement errors are also added (accumulated) together with measurement values and measurement errors are increased. As a result, in the case where associated information is presented based on the calculated current position and azimuth, there is a possibility that information not matching an actual current position or an actual current azimuth is presented and a user is got confused.
In order to solve the aforementioned problem in the autonomous navigation, the following method has been employed. That is, the position and the azimuth are calculated by several measurement methods and the measurement result is selected depending on the confidence levels of the calculation results. For example, in Patent Document 1, the reliability of the GPS positioning result is compared with the reliability of the autonomous navigation positioning result, and the positioning result of higher reliability is employed. As the index indicating the reliability of the GPS positioning result, the average value of HDOP information and SN information contained in GPS data is used. The index indicating the reliability of the autonomous navigation positioning result is calculated from the cumulative travel distance and the cumulative amount of direction change.